Shells brightly coloured/patterned to attract mates?

[ennss]

Thanks for your response.  Obviously, I'm merely a collector and enthusiaist  - I have no background in biology at all -  I didn't even think about the periostracum and lack of light at depth which are totally obvious now that you mention them.    So is there any other proposed purpose to the colour and pattern such as camouflage, or is it strictly a side effect during the deposition of the shell that has no purpose or "cost" to the animal?

The form of the shell on the other hand must be pretty directly related to some evolutionary  advantage, with heavier, thicker or spiny shells having additional protection from predators.  There's obviously a huge difference in "production cost" to the animal between very heavy thick shells and thin light shells...    Are all spines, frills etc. thought to have developed as direct protection from predators?